实现一个Promise之基础、异步

其实跟着promise a+一步一步，按照顺序实现一个promise并不难，今天先实现最简单的promise和异步resolve。本来想全部一次性写完，想想还是分开走，希望想学的可以敲几遍。

//promise三个状态

const PENDING = 'PENDING';

const FULFILLED = 'FULFILLED';

const REJECTED = 'REJECTED';



class Promise {

 constructor(executor){

  //状态

  this.state = PENDING;

  //成功值 失败值 默认undefined

  this.value = undefined;

  this.reason = undefined;

  //简易发布订阅，实例可以then多次，用数组

  this.onResolvedCallbacks = [];

  this.onRejectedCallbacks = [];

  

  //成功方法

  let resolve = (value) => {

   if(this.state === PENDING){

    this.state = FULFILLED;

    this.value = value;

    this.onResolvedCallbacks.forEach(fn => fn());

   }

  };

  

  //失败方法

  let reject = (reason) => {

   if(this.state === PENDING){

    this.state = REJECTED;

    this.reason = reason;

    this.onRejectedCallbacks.forEach(fn => fn());

   }

  };

  

  //立即执行，如果有错误直接执行失败方法

  try{

   executor(resolve, reject);

  }catch (e) {

   reject(e);

  }

 }

 //then方法，一个成功函数，一个失败函数

 then(onFulfilled, onRejected){

  //如果成功，执行成功函数

  if(this.state === FULFILLED){

   onFulfilled(this.value);

  };

  

  //如果失败，执行失败函数

  if(this.state === REJECTED){

   onRejected(this.reason);

  };

  

  //如果有异步，订阅成功失败函数

  if(this.state === PENDING){

   this.onResolvedCallbacks.push(() => {

    onFulfilled(this.value);

   });

   

   this.onRejectedCallbacks.push(() => {

    onRejected(this.reason);

   });

  };

 }

}

最简单的实现，可以理解理解。